1. Field of the Invention
The present invention relates to an apparatus for monitoring an optical signal-to-noise ratio, and more particularly to an apparatus for monitoring an optical signal-to-noise ratio in a WDM (Wavelength Division Multiplexing) optical network, using orthogonal polarization components of an optical signal.
2. Description of the Related Art
WDM technology is employed to transmit a number of independent optical channels, separated from each other, through a single optical fiber.
Although it provides relatively low transmission rates, use of the WDM scheme enables a significant increase in the transmission capacity per optical fiber, which is thus effective in achieving a broadband high-speed communication network.
In the WDM scheme, an optical amplifier for amplifying an optical signal is essential in compensating for loss in an optical fiber and increasing the transmission distance. However, the optical amplifier decreases the performance of an optical signal, because it reduces the optical signal-to-noise ratio due to amplified spontaneous emission noise occurring when the optical signal passes through the optical amplifier. Accordingly, if the optical signal-to-noise ratio directly related to the performance of the optical signal is measured, it is possible to measure the performance of the WDM system. If correct information of the performance of the WDM system is known, it is possible to operate, maintain and manage the WDM system more effectively.
FIG. 1 is a graph showing an optical spectrum after an optical signal passes through a number of optical add/drop multiplexers or optical cross-connects in a WDM optical network.
It can be seen from FIG. 1 that each channel has a different optical signal-to-noise ratio, and the spectrum of amplified spontaneous emission noise is not flat since the optical signal passes through a number of optical add/drop multiplexers or optical cross-connects.
In the prior art, an optical spectrum analyzer employing a rotating diffraction grating is used to measure the optical signal-to-noise ratio. The optical spectrum analyzer is advantageous in that it has a wide measurement range and a high accuracy, but disadvantageous in that it is expensive and has a large size.
Some optical SNR (signal-to-noise ratio) measurement methods have been proposed to make up for such a disadvantage of the optical spectrum analyzer.
One example of such optical SNR measurement methods employs an acousto-optic tunable filter. This method has a wide measurement range and a high-speed property, but has a low resolution, so it cannot be used when the channel spacing of WDM signals is narrow.
Another example uses an arrayed waveguide diffraction grating. This method infers the optical signal-to-noise ratio by measuring amplified spontaneous emission noise through a specific port other than signal measurement ports among output ports of the arrayed waveguide diffraction grating. However, this method employing an arrayed waveguide diffraction grating also has the same disadvantage as the former methods employing an optical spectrum analyzer or an acousto-optic tunable filter in that it can be used only when the amplified spontaneous emission noise spectrum is flat.
In other words, the prior arts employing the optical spectrum analyzer, the acousto-optic tunable filter or the arrayed waveguide diffraction grating, have a problem in that it is difficult to correctly measure the amount of amplified spontaneous emission noise of each WDM channel when the spontaneous emission noise spectrum is not flat.
A method has been proposed in recent, which can measure the optical signal-to-noise ratio in a WDM optical network while overcoming such a problem. In order to measure the optical signal-to-noise ratio for each channel, this method uses low frequency electrical noise occurring when detecting each optical channel. However, in order to perform the noise measurement, this method requires the PRBS(pseudo random bit sequences) length of a signal to be less than 215−1 bits. Accordingly, for signals such as real random data having a pattern length more than 215−1 bits, this method cannot measure low frequency noise due to the signal component larger than noise component. Thus, there is a need to provide a method that can measure the optical signal-to-noise ratio even for a signal whose amplified spontaneous emission noise spectrum is not flat, irrespective of the signal pattern length.